A sensor in an erratic state is characterized by rapidly changing readings. The conventional method for detecting an erratic sensor is to compute a variation value of the signal output from the sensor, and compare the variation value to a constant threshold. This method does not account for operating conditions which include expected rapid changes in the condition being sensed by the sensor, which when sensed by the sensor cause the sensor to output a rapidly fluctuating signal. Under these conditions, a false indication of an erratic temperature sensor may be given when the constant threshold value is set too low to account for rapidly changing operating conditions, and an erratic temperature sensor may not be detected when the constant threshold value is set higher to account for rapidly changing operating conditions.
A temperature sensor sensing the temperature of a power inverter module in a hybrid powertrain of a vehicle may output a rapidly fluctuating signal when sensing a rapid change in the temperature of the power inverter module, for example, during rapid changes in torque command input to the powertrain. In these operating conditions, use of a constant threshold value for diagnosis of an erratic state of an inverter temperature sensor will fail to distinguish between an inverter temperature sensor that is outputting a rapidly changing signal because it is in an erratic state, and one that is sensing expected rapid changes in temperature due to rapid changes in torque command input to the powertrain and inverter and emitting an expected rapidly fluctuating signal.